The specific aims of this proposal are to identify clinical criteria, including specific diagnostic tests, which best identify those patients who would profit from corpus callosum section; to measure the callosotomy; to develop improved criteria to select patients for each of these procedures; and to determine the immediate and long-term clinical, neuropsychological, and electroencephalographic consequences of corpus callosotomy. Corpus callosotomy is a promising therapeutic modality for the approximately one million people with epilepsy in the United States who are medically intractable. All too many patients do not have a discrete epileptiform focus in a surgically resectable region, but do suffer from rapid secondary generalization and frequent injuries from the resultant falling seizures or loss of awareness. We have performed 10 complete corpus callosotomies to date with highly significant reductions in both post operative generalized epileptiform discharges and falling seizures. We expect 30 patients to be candidates for the corpus callosotomy procedure during the three year study period. We propose to perform 10 more complete sections to identify those critical EEG, clinical, and neuronpsychological criteria to allow us to perform 20 partial callosotomies in the remaining two years of the study. Success of therapy will be measured by reduction of seizure frequency, ability to simplify antiepileptic medication regimens, and evidence of increased independence in daily living. Neuropsyhological effects will be studied in a separate proposal. Appropriate statistical analysis will be used as outlined in detail in the proposal.